Center and orient mesh of a head
A easy way would be:


Select three vertices (right, left and middle) on the head;
Create a empty based on that virtual plane using Precise Align.
Select the empty, the AltR. You may want to rotate manually along Z axis if it's not expected after that.
Select your object, AltP > Clear and Keep Transformation. Then delete the empty.




Similar usage like this.

One thing that is worthy mentioning is: it's a scanned head in your case. Since real human body can be just roughly symmetrical, but not exactly as it appears, so the result may not be perfectlly symmetrical as expected. Of course, you can still give it a try.